The present invention relates to medical and non-medical procedures in which it is desirable to measure and display the distribution of shear stresses within a subject in response to an external mechanical stimulus. Shear is defined as the change in velocity with respect to position. Shear is an important parameter in many mechanical systems. For example, with flowing fluids, shear (or shear rate) often determines the dynamic effects that the flow exerts on its surroundings. These effects include the rate of corrosion in pipes and the development of arteriosclerotic disease in blood vessels. Shear rates of non-flowing biological tissue is also an important parameter in the diagnosis of disease. The shear rate of a tissue is a measure of the tissues elasticity and elasticity is frequently used to asses tissue characteristics during physical examinations such as palpation. Although traditional fluid flow analysis methods such as ink streamlining and laser Doppler have been used to measure shear in moving fluids, these methods are invasive and are not suitable for in-vivo applications, particularly in the measurement of shear rates in non-flowing tissue.
Imaging of tissue elasticity under dynamic and static mechanical loads has been demonstrated with ultrasonic methods, as described in "Sono-elasticity: Medical elasticity images derived from ultrasound signals in mechanically vibrated targets." by Lemer, R. M. et al., Acoust. Imaging 1988, 16:317-327 and Tissue Response to Mechanical Vibrations For `SonoElasticity Imaging` by K. J. Parker, S. R. Huang, R. A. Musulin and R. M. Lerner 1990, 16:241-246, both hereby incorporated by reference. While providing a method for non-invasive imaging of sample elasticity, these methods require vast amounts of computing to track ultrasonic speckle patterns and suffer from all the limitations of ultrasonic imaging.
Since velocity is a vector quantity, it can be expressed as the sum of three mutually orthogonal component vectors. Each of these components, in turn, can be measured with respect to three mutually-orthogonal spatial dimensions to give a total of nine different shear measurements. Existing techniques can be used to measure some of these shear components, but detection of all components is difficult or impossible in most situations.